The mutagenic consequences of introducing defined lesions into DNA are being examined in a mutational system capable of detecting a wide spectrum of mutational events, both at and some distance away from the actual site of damage. Our efforts are focused on two lesions known to present major challenges to the stability of genetic information in animal cells, depurination and deamination. Both have been found to be highly mutagenic for base substitution errors, the former for transversions at purine bases, the latter for transitions at cytosine residues. Newly developed and highly sensitive genetic assays allow detection of spontaneous deamination mutagenesis using physiologically relevant conditions. Extension of this work to measurements of deamination rates within mispaired and extrahelical bases is currently in progress. This is the firt step in examining deamination due to cross strand protonation induced by DNA adducts in double stranded DNA. In a related study, the ability of the E. coli uvr ABC excinuclease to incise both normal and damaged DNA containing mispairs and extrahelical bases is being examined. These studies are intended to probe the structure and processing of both premutagenic adducts and the actual mispaired and misaligned intermediates which lead to base substitution, frameshift and deletion errors.